class calculator:
    def __init__(self, num):
        self.num = 5
    def exponent(self):
        return self.num*self.num
    def __getstate__(self):
        return "custom debug info: exponent is " + str(self.exponent())
f = calculator(5)
# normally, f: {a: 5} is shown
# but instead, we see f: {py/state: "custom debug info: exponent is 25"}